Κύριοι Νόμοι Φυσικής
Ampere's Law The line integral of the magnetic flux around a closed curve is proportional to the algebraic sum of electric currents flowing through that closed curve; or, in differential form curl B = J. This was later modified to add a second term when it was incorporated into Maxwell's equations. Archimedes' Principle A body that is submerged in a fluid is buoyed up by a force equal in magnitude to the weight of the fluid that is displaced, and directed upward along a line through the center of gravity of the displaced fluid. Avogadro's Hypothesis (1811) Equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules. It is, in fact, only true for ideal gases. Bernoulli's Equation In an irrotational fluid, the sum of the static pressure, the weight of the fluid per unit mass times the height, and half the density times the velocity squared is constant throughout the fluid. Biot-Savart Law A law which describes the contributions to a magnetic field by an electric current. It is analogous to Coulomb's law. Boyle's Law (1662); Mariotte's law (1676) The product of the pressure and the volume of an ideal gas at constant temperature is a constant. Bragg's Law (1912) When a beam of X-rays strikes a crystal surface in which the layers of atoms or ions are regularly separated, the maximum intensity of the reflected ray occurs when the complement of the angle of incidence, theta, the wavelength of the X-rays, lambda, and the distance between layers of atoms or ions, d, are related by the equation 2 d sin theta = n lambda, Brownian Motion (1827) The continuous random motion of solid microscopic particles when suspended in a fluid medium due to the consequence of ongoing bombardment by atoms and molecules. Casimir Effect A quantum mechanical effect, where two very large plates placed close to each other will experience an attractive force, in the absence of other forces. The cause is virtual particle-antiparticle pair creation in the vicinity of the plates. Also, the speed of light will be increased in the region between the two plates, in the direction perpendicular to them. Causality Principle The principle that cause must always preceed effect. More formally, if an event A ("the cause") somehow influences an event B ("the effect") which occurs later in time, then event B cannot in turn have an influence on event A. That is, event B must occur at a later time t than event A, and further, all frames must agree upon this ordering. Centrifugal Pseudoforce A pseudoforce on an object when it is moving in uniform circular motion. The "force" is directed outward from the center of motion. Charles' Law (1787) The volume of an ideal gas at constant pressure is proportional to the thermodynamic temperature of that gas. Cherenkov Radiation Radiation emitted by a massive particle which is moving faster than light in the medium through which it is traveling. No particle can travel faster than light in vacuum, but the speed of light in other media, such as water, glass, etc., are considerably lower. Cherenkov radiation is the electromagnetic analogue of the sonic boom, though Cherenkov radiation is a shockwave set up in the electromagnetic field. Complementarity Principle The principle that a given system cannot exhibit both wave-like behavior and particle-like behavior at the same time. That is, certain experiments will reveal the wave-like nature of a system, and certain experiments will reveal the particle-like nature of a system, but no experiment will reveal both simultaneously. Compton Effect (1923) An effect that demonstrates that photons (the quantum of electromagnetic radiation) have momentum. A photon fired at a stationary particle, such as an electron, will impart momentum to the electron and, since its energy has been decreased, will experience a corresponding decrease in frequency. Conservation Laws Conservation of mass-energy The total mass-energy of a closed system remains constant. Conservation of electric charge The total electric charge of a closed system remains constant. Conservation of linear momentum The total linear momentum of a closed system remains constant. Conservation of angular momentum The total angular momentum of a closed system remains constant. There are several other laws that deal with particle physics, such as conservation of baryon number, of strangeness, etc., which are conserved in some fundamental interactions (such as the electromagnetic interaction) but not others (such as the weak interaction). Constancy Principle One of the postulates of A. Einstein's special theory of relativity, which puts forth that the speed of light in vacuum is measured as the same speed to all observers, regardless of their relative motion. Continuity Equation An equation which states that a fluid flowing through a pipe flows at a rate which is inversely proportional to the cross-sectional area of the pipe. It is in essence a restatement of the conservation of mass during constant flow. Copernican Principle (1624) The idea, suggested by Copernicus, that the Sun, not the Earth, is at the center of the Universe. We now know that neither idea is correct. Coriolis Pseudoforce (1835) A pseudoforce which arises because of motion relative to a frame of reference which is itself rotating relative to a second, inertial frame. The magnitude of the Coriolis "force" is dependent on the speed of the object relative to the noninertial frame, and the direction of the "force" is orthogonal to the object's velocity. Correspondence Principle The principle that when a new, more general theory is put forth, it must reduce to the more specialized (and usually simpler) theory under normal circumstances. There are correspondence principles for general relativity to special relativity and special relativity to Newtonian mechanics, but the most widely known correspondence principle is that of quantum mechanics to classical mechanics. Coulomb's Law The primary law for electrostatics, analogous to Newton's law of universal gravitation. It states that the force between two point charges is proportional to the algebraic product of their respective charges as well as proportional to the inverse square of the distance between them. Curie's Law The susceptibility of an isotropic paramagnetic substance is related to its thermodynamic temperature T by the equation KHI = C / T. Curie-Weiss Law A more general form of Curie's Law, which states that the susceptibility of a paramagnetic substance is related to its thermodynamic temperature T by the equation KHI = C/T - W, where W is the Weiss constant. Dalton's Law of partial pressures The total pressure of a mixture of ideal gases is equal to the sum of the partial pressures of its components; that is, the sum of the pressures that each component would exert if it were present alone and occupied the same volume as the mixture. Doppler Effect Waves emitted by a moving object as received by an observer will be blueshifted (compressed) if approaching, redshifted (elongated) if receding. It occurs both in sound as well as electromagnetic phenomena. Dulong-Petit Law (1819) The molar heat capacity is approximately equal to the three times the ideal gas constant: Einstein Field Equation The cornerstone of Einstein's general theory of relativity, relating the gravitational tensor G to the stress-energy tensor T by the simple equation G = 8 pi T. Einstein's Mass-Energy Equation The energy E of a particle is equal to its mass M times the square of the speed of light c, giving rise to the best known physics equation in the Universe: E = M c2. Equivalence Principle The basic postulate of A. Einstein's general theory of relativity, which posits that an acceleration is fundamentally indistinguishable from a gravitational field. Faraday's Law The line integral of the electric field around a closed curve is proportional to the instantaneous time rate of change of the magnetic flux through a surface bounded by that closed curve; in differential form curl E = -dB/dt, where here d/dt represents partial differentiation. Faraday's Laws of electrolysis Faraday's first law of electrolysis The amount of chemical change during electrolysis is proportional to the charge passed. Faraday's second law of electrolysis The charge Q required to deposit or liberate a mass m is proportional to the charge z of the ion, the mass, and inversely proportional to the relative ionic mass M; mathematically Q = F m z / M, Faraday's first law of electromagnetic induction An electromotive force is induced in a conductor when the magnetic field surrounding it changes. Faraday's second law of electromagnetic induction The magnitude of the electromotive force is proportional to the rate of change of the field. Faraday's third law of electromagnetic induction The sense of the induced electromotive force depends on the direction of the rate of the change of the field. Fermat's Principle The principle states that the path taken by a ray of light between any two points in a system is always the path that takes the least time. Gauss' Law The electric flux through a closed surface is proportional to the algebraic sum of electric charges contained within that closed surface; in differential form div E = rho, where rho is the charge density. Gauss' Magnetic Law no magnetic charges exist; in differential form div B = 0. Hall Effect When charged particles flow through a tube which has both an electric field and a magnetic field (perpendicular to the electric field) present in it, only certain velocities of the charged particles are preferred, and will make it un-deviated through the tube; the rest will be deflected into the sides. Hooke's Law The stress applied to any solid is proportional to the strain it produces within the elastic limit for that solid. The constant of that proportionality is the Young modulus of elasticity for that substance. Huygens' Principle The mechanical propagation of a wave (specifically, of light) is equivalent to assuming that every point on the wavefront acts as point source of wave emission Ideal Gas Law An equation which sums up the ideal gas laws in one simple equation P V = n R T, Joule-Thomson Effect; Joule-Kelvin Effect The change in temperature that occurs when a gas expands into a region of lower pressure. Joule's Laws Joule's first law The heat Q produced when a current I flows through a resistance R for a specified time t is given by Q = I2 R t . Kirchhoff's Rules loop rule The sum of the potential differences encountered in a round trip around any closed loop in a circuit is zero. point rule The sum of the currents toward a branch point is equal to the sum of the currents away from the same branch point. Kohlrausch's Law If a salt is dissolved in water, the conductivity of the solution is the sum of two values -- one depending on the positive ions and the other on the negative ions Lambert's Laws Lambert's first law The illuminance on a surface illuminated by light falling on it perpendicularly from a point source is proportional to the inverse square of the distance between the surface and the source. Lambert's second law If the rays meet the surface at an angle, then the illuminance is proportional to the cosine of the angle with the normal. Lambert's third law The luminous intensity of light decreases exponentially with distance as it travels through an absorbing medium. Laplace Equation For steady-state heat conduction in one dimension, the temperature distribution is the solution to Laplace's equation, which states that the second derivative of temperature with respect to displacement is zero. Lenz's Law (1835) An induced electric current always flows in such a direction that it opposes the change producing it. Mach Number The ratio of the speed of an object in a given medium to the speed of sound in that medium. Mach's Principle (1870) The inertia of any particular particle or particles of matter is attributable to the interaction between that piece of matter and the rest of the Universe. Thus, a body in isolation would have no inertia. Maxwell's Equations (1864) Gauss' law The electric flux through a closed surface is proportional to the algebraic sum of electric charges contained within that closed surface; in differential form div E = rho, where rho is the charge density. Gauss' law for magnetic fields The magnetic flux through a closed surface is zero; no magnetic charges exist. In differential form div B = 0. Faraday's law The line integral of the electric field around a closed curve is proportional to the instantaneous time rate of change of the magnetic flux through a surface bounded by that closed curve; in differential form curl E = -dB/dt,.. Ampere's law, modified form The line integral of the magnetic field around a closed curve is proportional to the sum of two terms: first, the algebraic sum of electric currents flowing through that closed curve; and second, the instantaneous time rate of change of the electric flux through a surface bounded by that closed curve; in differential form curl H = J + dD/dt,. In addition to describing electromagnetism, his equations also predict that waves can propagate through the electromagnetic field, and would always propagate at the the speed of light in vacuum. Murphy's Law (1942) If anything can go wrong, it will. Newton's Law of universal gravitation Two bodies attract each other with equal and opposite forces; the magnitude of this force is proportional to the product of the two masses and is also proportional to the inverse square of the distance between the centers of mass of the two bodies; F = (G m M/r2) e, where m and M are the masses of the two bodies, r is the distance between. the two, and e is a unit vector directed from the test mass to the second. Newton's Laws of motion Newton's first law of motion A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force. Newton's second law of motion For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed; the constant of proportionality is the inertial mass of the body. Newton's third law of motion In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction force. Occam's Razor (1340) If two theories predict phenomena to the same accuracy, then the one which is simpler is the better one. Moreover, additional aspects of a theory which do not lend it more powerful predicting ability are unnecessary and should be stripped away. Ohm's Law (1827) The ratio of the potential difference between the ends of a conductor to the current flowing through it is constant; the constant of proportionality is called the resistance, and is different for different materials. Pascal's Principle Pressure applied to an enclosed incompressible static fluid is transmitted undiminished to all parts of the fluid. Peter Principle In a hierarchy, every employee tends to rise to his level of incompetence. Planck Equation The quantum mechanical equation relating the energy of a photon E to its frequency nu: E = h nu. Reflection Law, Snell's Law For a wavefront intersecting a reflecting surface, the angle of incidence is equal to the angle of reflection, in the same plane defined by the ray of incidence and the normal. Refraction Law For a wavefront traveling through a boundary between two media, the first with a refractive index of n1, and the other with one of n2, the angle of incidence theta is related to the angle of refraction phi by n1 sin theta = n2 sin phi. Relativity Principle The principle, employed by Einstein's relativity theories, that the laws of physics are the same, at least qualitatively, in all frames. That is, there is no frame that is better (or qualitatively any different) from any other. This principle, along with the constancy principle, constitute the founding principles of special relativity. Stefan-Boltzmann Law The radiated power P (rate of emission of electromagnetic energy) of a hot body is proportional to the radiating surface area, A, and the fourth power of the thermodynamic temperature, T. The constant of proportionality is the Stefan-Boltzmann constant. Mathematically P = e sigma A T4,.where the efficiency rating e is called the emissivity of the object. Superposition Principle The general idea that, when a number of influences are acting on a system, the total influence on that system is merely the sum of the individual influences; that is, influences governed by the superposition principle add linearly. Thermodynamic Laws First law of thermodynamics The change in internal energy of a system is the sum of the heat transferred to or from the system and the work done on or by the system. Second law of thermodynamics The entropy -- a measure of the unavailability of a system's energy to do useful work -- of a closed system tends to increase with time. Third law of thermodynamics For changes involving only perfect crystalline solids at absolute zero, the change of the total entropy is zero. Zeroth law of thermodynamics If two bodies are each in thermal equilibrium with a third body, then all three bodies are in thermal equilibrium with each other. Uncertainty Principle (1927) A principle, central to quantum mechanics, which states that two complementary parameters (such as position and momentum, energy and time, or angular momentum and angular displacement) cannot both be known to infinite accuracy; the more you know about one, the less you know about the other. van der Waals force Forces responsible for the non-ideal behavior of gases, and for the lattice energy of molecular crystals. There are three causes: dipole-dipole interaction; dipole-induced dipole moments; and dispersion forces arising because of small instantaneous dipoles in atoms. Wave-Particle Duality The principle of quantum mechanics which implies that light (and, indeed, all other subatomic particles) sometimes act like a wave, and sometime act like a particle, depending on the experiment you are performing. For instance, low frequency electromagnetic radiation tends to act more like a wave than a particle; high frequency electromagnetic radiation tends to act more like a particle than a wave. Wiedemann-Franz Law The ratio of the thermal conductivity of any pure metal to its electrical conductivity is approximately constant for any given temperature. This law holds fairly well except at low temperatures. Κατηγορία:Νόμοι Φυσικής